1. Field of the Invention
This invention relates generally to devices for forming aerosols and especially to thermal combustion aerosol devices, also known as foggers.
2. State of the Art
It is known in the art to apply sprout inhibitors of various types to potatoes to prevent sprouting during storage. One of the earlier patents relating to this is the Plant patent, U.S. Pat. No. 3,128,170, related to a method for applying isopropyl-N-chlorophenylcarbamate (CIPC) to a potato storage facility. As noted in the Plant patent, CIPC is a solid at room temperature which is generally dissolved in polar solvents such as propylene glycol and, more recently, methanol. A typical weight of CIPC in solution is about 78% of the weight of the solution for commercial products using methanol as the solvent. A solution of CIPC has generally been desirable for forming aerosols of CIPC as well as for handling.
More recent patents relating to the application of CIPC to a storage facility are Sheldon and Morgan, respectively, U.S. Pat. Nos. 4,226,179 and 4,887,525. Sheldon involves a process for ultrasonically vaporizing a solution of CIPC, while the Morgan patent relates to an improved technique for moderating the air flow within a storage facility to get better distribution of CIPC aerosol produced from solvent-based system.
Sheldon refers to the possibility of applying non-solvent based CIPC. No example of such technique is given in Sheldon nor is there any suggestion as to how such a technique would be accomplished. Sheldon indicates that a solvent may be necessary in order to keep the chemical liquid in the spray nozzle.
Sheldon notes that thermal fogging tends to produce large droplets of CIPC, cause degradation of the CIPC and warm the stored potatoes, which may promote bacteria growth. Thus, Sheldon was directed to a non-thermal or low-thermal application in contrast to the thermal fogging apparatus in common use.
The technique utilized in the Morgan patent involved thermal fogging prior to introduction of the fog of CIPC in the circulating air stream of a storage facility. Thermal foggers which have been commercially used are constructed similar to that illustrated in FIG. 12, wherein a propane flame burns within a hollow pipe (combustion chamber) which is enclosed by another generally cylindrical enclosure. It is into this outer enclosure that the solvent based CIPC is introduced. The solvent based CIPC is frequently introduced near the distal end of the combustion chamber with the solvent solution of CIPC being blended with the combustion gases emanating from the combustion chamber. This results in the solvent generally being evaporated and the CIPC being converted into a mixture of vapor (gas) and particles of CIPC, both liquid and solid particles.
The products of combustion exiting the combustion chamber are generally oxygen poor, so much of the methanol solvent is not burned. Thus, it evaporates and often decomposes to formaldehyde and formic acid, both of which are toxic. The products of combustion form a reducing atmosphere in a storage facility and further create an over pressure from the large volume of gases entering the facility. The reducing atmosphere causes the potatoes to be stressed, resulting in some of the starch being converted to sugars. Potatoes having a high sugar content yield french fries which are dark brown in color when cooked, especially at the tips of the french fries. This is generally undesirable and reduces the value of such stored potatoes. An over pressure results in much of the treatment chemical being vented from the storage facility.
As the Morgan patent noted, one of the problems had been that the CIPC tended to collect on the fans of the air circulation system of the storage facility as well as on the vent pipes and other portions of the facility. CIPC is not really effective for treating potatoes unless it is in contact with the potatoes, that is, deposited directly on the potatoes. The prior art thermal fogging system introduces into the potato storage facility all the products of combustion of the propane gas burner as well as evaporated methanol, or such other solvent, including decomposition products of methanol such as formaldehyde, formic acid and the like. Given that storage facilities are maintained at relatively low temperatures, in the neighborhood of about 40 to 50.degree. F., these products, methanol, formic acid, formaldehyde, and the like, can liquify (condense) within the facility and can also be deposited on the potatoes. Since this can happen, it can also create a vapor pressure of these products within the storage facility long after a sprout inhibition treatment has occurred. Thus, a facility can be rendered unsafe for personnel to work in for quite some time. Although thermal fogging with thermal foggers of the type illustrated in FIG. 12 has been done for a long time, and storage sheds so treated have been relatively free of sprout growth, nevertheless the method is inefficient in its application of CIPC, i.e., CIPC decomposes to some extent, contaminates the storage facility with toxic, undesirable material, and is lost by venting.
Both Sheldon and Morgan involve methods and apparatus which introduce large volumes of gases, air or combustion products into a storage facility. This creates an over pressure within the facility and causes venting and loss of CIPC from the facility.
The technique of forming aerosols, i.e., a stable fog, of herbicides, pesticides, etc. has conventionally involved the use of solvents or carriers. In U.S. Pat. No. 2,460,792 of Pabst et al., the technique of adding a mixture of oils to obtain a stable aerosol is disclosed. A principal reason for the use of solvents with CIPC and similar sprout inhibitors apparently has been to accommodate the application of the sprout inhibitor as an aerosol and to facilitate handling of liquids by applicators.
While the Sheldon patent suggests the forming of a liquid particle "fog" by ultrasonic means, the technique has apparently not been practiced commercially and the patent is devoid of any instruction as how this is done from a non-solvent system. Current field techniques for commercial application of CIPC has been by fogging of a solvent solution of CIPC via a thermal fogger of the prior art type, frequently using moderated fan speed as taught by the Morgan patent.
While effort has been expended towards improving the materials to be converted to an aerosol, little effort has heretofore been made to improve the thermal fogging apparatus to be more efficient.